This project investigates primate biobehavioral development through comparative longitudinal investigations, with special emphasis on studying individual differences among rhesus monkeys in response to mild environmental challenge and on determining their long-term developmental consequences in different physical and social environments. Findings of particular interest from studies completed this past year include the following: (1) Standardized measures of rhesus monkey neonatal reflex, tone, and state change patterns obtained during the first month of life clearly differentiated subjects on the basis of rearing environment and best predictor of optimal performance on cognitive tests administered 6-8 months later. In addition, the cognitive tests revealed positive consequences of early "enrichment" within each rearing environment studied. (2) Ongoing studies of biobehavioral continuity and change from birth to adulthood yielded long-term effects on both behavioral and physiological systems attributable to differential rearing experiences during the first 6 months of life. However, monkeys within each rearing condition were highly stable in terms of individual differences in behavioral, physiological, and immunological measures in response to mild challenge, even in the face of major developmental changes for most of these measures. (3) Comparisons of behavioral, neuroendocrine, and immunological response profiles between breeding colony males and their multiple offspring revealed striking cross-generational similarities in pattern of response, despite the fact that the males never were exposed to these offspring. These findings add to a growing body of evidence suggesting that such patterns are highly heritable. (4) Preliminary data from a new study of response to challenge in a wild rhesus monkey troop revealed patterns of neuroendocrine and behavioral response virtually identical to those reported in laboratory studies of captive-born monkeys, indicating considerable generality of the previous laboratory-based findings and additionally suggesting a viable setting in which to assess the relative adaptive fitness of these different response patterns.